Rechearch Tec, what one?

You should be able to colonise any planet with that tech set the atmosphere type wont stop you. However you will find that worlds with unbreathable atmospheres will give rise to domed colonies which have less capacity for facilities storage and population than a colony on a similar world with a breathable atmosphere. There is as far as I know no tech for colonisation by atmosphere type in the main game.

You can colonize any planet that has a red or green ring by it. The green ones have your type of atmosphere and the rest either have a different or no atmosphere. As noted above, "red ring" colonies will be domed and you'll get 1/5 of the facilities space you would in the open air equivalent.

You can research atmosphere processing centers as well. Research planetary engineering until you get planet utilization. Then get to L20 in the tech and you'll get the facility that'll enable you to change a planet's atmosphere to breathable in 10 years. You can build multiple ones, so it doesn't have to take quite as long to crack open those domes.

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"No plan survives contact with the enemy"
Moltke

actually theres a setting when you start the game and you get to choose weather or not you can colonize "red flagged" planets. so if you cant thats the setting you (probably accedently) chose.

My solution is to make treaties allowing migration of other races to my empire - races that breathe the appropriate atmospheres. Then I just create empty colonies (ie, no colonists) of the right atmosphere, and they flock there on their own. Once you have one such colony, you can then use it to colonize other planets with that atmosphere.

People who migrate in that way are automatically part of your empire, they don't need to assimilate. I'm not certain, but I have the impression that when you conquer planets of a given atmosphere, moving some loyalists who breathe the same atmosphere there improves the assimilation of the subjugated.